Increased expression of the Fibroblast growth factor 8 (Fgf8) gene apparently plays an important role in the progression of both breast and prostate cancer. To understand how abnormal Fgf8 expression affects cell function, we are studying its normal role during vertebrate embryogenesis, using the mouse as a model system. Fgf8 is expressed in a variety of regions of the embryo that may be termed "organizers": regions that are a source of signals that pattern and thus "organize" the surrounding tissue. An allelic series generated at the Fgf8 locus (Meyers et al. 1998 Nature Genetics 18:136), as well as Cre-mediated tissue-specific knockouts (Lewandoski et al. 2000 Nature Genetics, 26:460; Lewandoski 2001 Nature Reviews Genet. 2:743) has revealed a role for Fgf8 in organizers that control gastrulation, limb, and brain development. Recently we have produced a valuable mouse line that expresses Cre throughout all embryonic mesoderm/endodermal lineages but not neuroedctoderm, thus allowing us to control gene expression in these lineages. This line is useful to bypass the embryonic lethal phenotypes of genes that affect early development, yet allows the study of the role of such genes throughout much of the embryo. Inactivation of Fgf8 with TCre has revealed that Fgf8 plays a central role in cell survival and gene expression during kidney development. One of the intriguing insights that has emerged from these studies is that at different stages of embryogenesis FGF signaling plays different roles in cell migration, proliferation, patterning, and survival. How is this diversity of response achieved? To answer this question, we are studing downstream targets of FGF signaling. One set of such target genes is the homeobox genes Gbx1 and 2. The role of the mouse Gbx2 gene during neurulation and particularly in defining the mid/hindbrain organizer has been well documented (Wasserman et al. 1997 Development 124:2923). We are currently extending this analysis by studying a hypomorphic (partial-loss-of-function) Gbx2 allele, which has revealed that Gbx2 is required at certain threshold levels for different parts of the brain. Compared to Gbx2 relatively little has been reported about Gbx1. We recently described the cloning and embryonic expression pattern of Gbx1 and defined regions of potential molecular redundancy with Gbx2. (Waters et al 2003 Gene Exp. Patterns. 3:313). We are currently generating mice with an allelic series at the Gbx1 locus to study it's function during development, including its role in FGF signaling.